1. Field of the Invention
The present invention relates to turbine engines and, more particularly, pertains to axial and centrifugal compressors for such engines.
2. Description of the Prior Art
Compressors are used in gas turbine engines to compress and direct pressurized gas, such as air, to a combustion section of a gas turbine engine. Such compressors generally comprise a rotor assembly, carrying a number of radially extending blades, mounted on an axis for rotation within a stationary casing or shroud. The rotation of the rotor assembly causes the gas to be drawn into the rotor assembly and to be discharged at high pressure to the combustion section of the gas turbine engine.
It is a characteristic, common to such compressors, to surge under certain operating conditions. Typically, surge results from unstable air flow condition within the compressor. Compressor surge is generally characterized by a complete flow breakdown or flow reversal through the compressor, or by a sharp reduction of the airflow handling ability of the engine for its operating rotational speed. The value of airflow and pressure ratio at which a surge occurs is referred to as surge point. The surge line, which joins all the surge points, defines the maximum stable airflow which can be obtained at any rotational speed. A compressor must be designed to have a good safety margin between the airflow and pressure ratio at which it will be operated and the airflow and pressure ratio at which a surge will occur.
Accordingly, various attempts have been made to increase the surge margin under all operating conditions. For instance, U.S. Pat. No. 4,086,022 issued on Apr. 25, 1978 to Freeman et al. and U.S. Pat. No. 5,137,419 issued on Aug. 11, 1992 to Waterman both disclose a compressor casing having an inner surface defining a plurality of circumferentially spaced-apart slots adjacent the tips of a row of rotor blades. The slots are sized and positioned such as to extend in part upstream or downstream of the blades.
Although the compressor casings described in the above mentioned patents are effective for improving surge margin, they induce relatively a large radial displacement of gas flow which results in pressure loss.